Mixing two gaseous streams is a very common process in a variety of industrial applications. Examples include oxygen enrichment of a premixed fuel and air stream or oxygen enrichment of secondary, tertiary and over-fire air streams in industrial furnaces and boilers. Oxygen enrichment of gaseous fuel streams has been successfully practiced in a number of industrial processes, such as glass melting, lime and cement kilns, and steel manufacturing, among many others. Coal, on the other hand, is the most abundant fossil fuel currently available. Most of the power generated in the world uses coal as the fuel.
The enrichment of a particle-laden stream, such as a coal/air stream, with oxygen poses additional challenges. These challenges arise due to a large number of factors, as described below.
Firstly, due to safety concerns, high oxygen concentrations close to the walls of the burner primary duct or oxygen lance should be avoided.
Secondly, the coal particles usually have a non-uniform particle size distribution. Most utilities, with coal-fired power plants, use a size fraction that ranges from about 75-120 μm. The coal particle trajectories in the particle-laden stream deviate from the gas streamlines. The larger particles deviate more, whereas the smaller particles follow gas streamlines more closely. In addition, the bends in the line lead to a phenomenon known as “rope phenomena.” As a result, the particle loading is not uniform across the cross-section of the pipe.
Another challenge is the effect of oxygen injection on the particle trajectories. The injection may disturb these trajectories and could lead to a non-uniform loading of the particles in the stream. This could lead to pockets that are fuel lean. This, in turn, has a detrimental effect on the NO, emissions, which tend to increase under such conditions.
Non-uniform distribution of particles downstream the oxygen injection nozzle could lead to fuel lean pockets/zones at the burner primary air duct outlet. This, in turn, has a detrimental effect on the NO, emissions, which tend to increase under such conditions. This negative effect of non-uniform particles loading could be avoided if good oxidant/fuel mixing are achieved. In other words, the oxygen distribution should match the fuel distribution at the burner primary air duct outlet. That way, and in the first stages of coal combustion, oxygen and particles will be able to closely react during coal devolatilization (this reaction is required to decrease the NOx emissions).
Avoiding local increase of axial velocity at the burner outlet is a new challenge raised by oxygen injection. As increased residence times of both oxygen and fuel particles in the hot fuel rich zone are key parameters governing NOx emissions reduction, accelerating the flow at the burner outlet will lead to NOx emissions increase.
Currently, there are a number of injector designs that are used for mixing of two gaseous streams, including that disclosed in U.S. Pat. No. 5,356,213, the contents of which are incorporated herein by reference. This injector design, promoted by Air Liquide under the name Oxynator®, is designed to minimize mixing distance and to prevent high oxygen concentrations near the pipe walls. Oxygen flows radially from the center of the pipe and tangentially swirls in the air stream, improving mixing efficiency.
Other designs include those having a number of nozzle openings in order to efficiently mix two gaseous streams. Typical examples are disclosed in U.S. Pat. Nos. 5,664,733, 5,775,446, and 5,762,007.
For the coal-air flow, it is important to understand that the primary air is surrounded by a secondary air stream, which is injected in the furnace with a swirl to achieve mixing of the secondary air with the primary coal-air stream. The design of these burners is often relatively complex. New designs by Babcock and Wilcox now propose a transition zone between the primary and secondary air streams. These burners, however, must be retrofitted with some injection devices to enrich the primary coal-air stream with oxygen.
Particular pulverized coal burners are disclosed in U.S. Pat. No. 4,556,384. This patent shows various methods for injecting oxygen to enrich the primary coal-air stream. This burner is designed to operate in a stable manner with no risk of explosion with pure oxygen or air highly enriched with oxygen as the reactant. Streams of oxygen and fuel (pulverized coal) are delivered through concentric pipes (concentric tubes and annular rings).
Some other designs are those that are disclosed by U.S. Published Patent Application No. 2004-0074427 A1.
Additionally, four particular lance designs were proposed in U.S. Published Patent Application No. 2004-0185404 A1, which was published on Sep. 23, 2004, and contents of which are incorporated herein by reference. These recent designs, however, suffer from the drawbacks that the oxygen injection lead to a particle-depleted zone and does not allow for complete mixing of oxygen with the coal-air streams.